The present invention relates generally to a shift control system for an automatic transmission of a vehicle. More particularly, the present invention relates to a shift control system for controlling a lock-up clutch of a torque converter.
As is well known, a torque converter for an automatic transmission includes a lock-up clutch by way of which an input member is connected to an output member of the torque converter. The operative state that torque is transmitted with some slippage of the lock-up clutch is here referred to as a half-lock-up state. Thus, it is preferable to control slippage of the lock-up clutch not only for preventing uncomfortable vibration and noise but also for improving fuel economy. A feedback control system for executing half-lock-up control of the lock-up clutch is described in Japanese Patent Publication NO. 63-13060. According to this prior invention, an engaging force for engaging the lock-up clutch is controlled to set in a proper value corresponding to the difference between a pump speed (i.e., an engine speed) and a turbine speed.
The lock-up clutch arranged in the torque converter of the automatic transmission is generally engaged by oil pressure. Thus, in the feedback control system as disclosed in Japanese Patent Publication NO. 63-13060, the oil pressure applied to the lock-up clutch is controlled by feedback control so as to maintain slippage of the lock-up clutch in a predetermined target value. In FIG. 7, a half-lock-up range is shown in a shift pattern diagram.
In the above-mentioned feedback control system, excessive slippage is prevented so that t he fuel economy does not fall and durability of a friction element does not deteriorate. In addition, a quantity of slippage of the lock-up clutch does not excessively small so that booming noise and vibration due to fluctuation of an engine torque are prevented.
Therefore, the aforementioned advantages can be obtained by executing half-lock-up control for the lock-up clutch. However, the half-lock-up control for the lock-up clutch is not always executed regardless of the present running state of the vehicle. In practice, the half-lock-up control for the lock-up clutch is inhibited by various kinds of conditions of the vehicle.
For example, when the present engine speed is lower than a predetermined value, vibration is liable to arise. At this time, the half-lock control for the lock-up clutch is inhibited. Also in case that a temperature of an oil in the automatic transmission is high, since the oil temperature is additionally elevated due to slippage of the lock-up clutch, the half-lock-up control for the lock-up clutch is also inhibited. In addition, in case that warming-up of an engine is not still completed or in case that data (a rotational speed of the pump, a rotational speed of the turbine or the like) required for execution of the half-lock-up control for the lock-up clutch are not obtainable due to failure of sensors, the half-lock-up control for the lock-up clutch is likewise inhibited.
Among many inhibitive conditions as mentioned above, there is included a condition that the half-lock-up control for the lock-up clutch is inhibited due to reduction of an engine speed in excess of a predetermined value. Generally, while a vehicle runs at a high speed using a higher speed stage such as an overdrive stage, there arises a case that the half-lock-up control for the lock-up clutch is inhibited due to reduction of an engine speed.
There is a general tendency that fuel economy is improved when the vehicle runs with reduction of an engine speed while a speed stage is set to a higher one. In case that the half-lock-up control for the lock-up clutch is inhibited due to a low engine speed, it is recommendable to execute the half-lock-up control for the lock-up clutch by shifting to a lower speed stage in order to improve fuel economy.
However, as far as the conventional automatic transmission is concerned, a shift stage is determined in consideration of an engine speed, a vehicle speed or the like but it is not determined dependent on whether half-lock-up control for the lock-up control can be executed or not. Lately, there is tendency that a total gear ratio of the vehicle is elevated to a higher one, causing an engine speed at a higher speed stage to be reduced. For this reason, with the conventional automatic transmission, there are many cases that half-lock-up control for the lock-up clutch is inhibited during running of the vehicle with a higher speed stage. In other words, there are many cases that the vehicle runs with a higher speed stage while the lock-up clutch is released from the engaged state. As a result, there arises a problem that fuel economy is deteriorated.